1. Field of the Invention
The present invention relates to an optical pickup device for use in an optical disk apparatus for optical recording/reproduction of information on recording media such as optical disks, and a hologram laser unit to be incorporated in such an optical pickup device. More particularly, the present invention relates to an optical pickup device which is capable of performing accurate recording/reproducing operations for a plurality of types of optical disks having different recording densities; and a hologram laser unit to be incorporated in such an optical pickup device.
2. Description of the Related Art
In recent years, optical disks capable of recording signals representing a large amount of information at a high density have been utilized in a number of fields including audio, video, and computer applications. Compact disks (CDs), video disks, mini-disks (MDs), and magneto-optical disks for computers, which are widely commercially available now, generally employ a substrate which is 1.2 mm thick. Accordingly, optical pickup devices for use in performing recording and reproducing operations of information for these optical disks are designed so as to correct the aberration due to the 1.2 -thick substrate by utilizing an objective lens incorporated in the optical pickup devices.
A variety of attempts have been made to enhance the recording capacity of optical disks. Among these attempts is a method of improving the optical resolution by employing an objective lens having an increased numerical aperture (NA), and a method of recording or reproducing information by employing light (laser light) having a shorter wavelength.
The diameter xcfx86 of a converged beam spot on an optical disk can be represented by the following formula:
xcfx86=Kxc3x97xcex/NA
where K is a constant; NA is the numerical aperture of the objective lens; and xcex is the wavelength of a laser beam used. According to the above equation, the converged beam diameter xcfx86 decreases in inverse proportion to the numerical aperture NA. On the other hand, tolerance for a tilt of the optical disk decreases in proportion to the numerical aperture of the objective lens to the power of 3 (i.e., NA3). Therefore, in order to increase the numerical aperture NA of the objective lens while maintaining the same level of tolerance for the tilt of the optical disk, it is necessary to reduce the thickness of the substrate of the optical disk. Specifically, when the numerical aperture NA of the objective lens is increased from 0.5 to 0.6, for example, the thickness of the substrate (i.e., the substrate thickness) of the optical disk is required to be reduced to approximately 0.6 mm in order to maintain the same level of tolerance for the tilt of the disk as that of an optical disk having a 1.2 -thick substrate.
Thus, in order to realize a higher-density optical disk, it is necessary to optimize both the substrate thickness of the optical disk and the numerical aperture (NA) of the objective lens used in the optical pickup device.
However, if the substrate thickness of the optical disk is reduced as described above, the optical disk is no longer compatible with optical disks having a substrate of a conventional thickness. Accordingly, it becomes necessary to adjust the numerical aperture (NA) of the objective lens in the optical pickup device in accordance with the substrate thickness of the optical disk.
In this connection, for example, Japanese Laid-Open Publication No. 8-45105 discloses a method of adaptably changing the numerical aperture NA of the objective lens.
Specifically, the above laid-open publication describes means for selectively changing an aperture of an objective lens (hereinafter referred to as xe2x80x9cselective aperture changing meansxe2x80x9d) which is provided integrally with a moving unit for the objective lens. The selective aperture changing means is configured so that the aperture is effectively changed by inserting into or retracting from the optical path a plate for restricting the aperture, or by partially varying the transmittance of a liquid crystal plate. The use of such selective aperture changing means makes it possible to adaptably change the numerical aperture NA of the objective lens in the optical pickup device according to different substrate thicknesses of the optical disk.
Furthermore, Japanese Laid-Open Publication No. 6-124477 discloses a method of changing the numerical aperture NA using a liquid crystal filter and a polarizing filter.
Specifically, as described in the above laid-open publication, a predetermined pattern of electrodes are attached to a liquid crystal filter. For example, the electrodes may be patterned into an inner portion and an outer portion defining concentric circles. By applying a voltage to the respective portions of the patterned electrodes with appropriate timing, some portions of the filter are imparted with a polarization direction which is rotated by 90 degrees from that of the other portions. As a result, light which has passed through the liquid crystal filter is selectively divided into portions having different polarization states. When light having such portions of selectively varied polarization states is allowed to pass through a polarization beam splitter, the light is divided into a reflection light component and a transmittance light component depending on the polarization states. By configuring the optical system so that only the transmittance light component enters the objective lens, the diameter of the light beam entering the objective lens can be varied. As a result, the numerical aperture NA of the objective lens can be effectively changed.
However, the various aforementioned conventional methods of changing the numerical aperture NA have the following problems.
The method in which the aperture restriction plate is placed in and out of the optical path requires a mechanism capable of highly precisely performing the insertion and retraction of the aperture restriction plate, resulting in an increase in the size and manufacturing cost of the device thereby adversely affecting mass production. Moreover, since the insertion/retraction mechanism is required to be integrated and driven with the moving unit for the optical pickup, the mass of the moving unit is inevitably increased so as to degrade the servo performance of an actuator used for driving the moving unit.
The method using the liquid crystal plate does not require any additional moving mechanism. However, in the case where the liquid crystal plate is integrated with the moving unit for the optical pickup device, the whole size of the moving unit is increased. Furthermore, a movable mechanism for supplying operating power is required to securely supply a necessary voltage to the electrodes attached to the liquid crystal plate as the liquid crystal plate is moved along with the moving unit.
Alternatively, in the case where the liquid crystal plate is provided separately from the moving unit, the optical axis of the lens may be misaligned with respect to the center of the pattern of the liquid crystal plate as the objective lens is driven. Moreover, regardless of whether the liquid crystal plate is integrated with the moving unit or not, the liquid crystal materials contained in the liquid crystal plate are susceptible to a change in their characteristics (e.g., refractive index) due to a change in temperature. Therefore, such a device may not exhibit the desired performance due to changes in the environmental conditions.
In addition, all of the above-described methods require an additional mechanism in the optical pickup device, likely causing drawbacks such as complicated configuration and increase in cost.
A hologram laser unit of the present invention is to be used for performing recording and reproducing operations of information for an optical disk. The hologram laser unit includes a light source, a photodetector, and a hologram element which are formed integrally with each other. The hologram element includes: a first hologram for detecting information signals from the optical disk, the first hologram being provided in an area having an effective diameter corresponding to a numerical aperture suitable for the optical disk; and a second hologram for compensating for only an transmitted light amount of light traveling toward the optical disk from the light source, the second hologram being provided contiguously outside the first hologram.
Directions of diffraction of the first and second holograms may be substantially orthogonal with each other.
Preferably, there is no substantial difference between a pitch of the first hologram and a pitch of the second hologram.
The hologram laser unit may further include a second photodetector corresponding to the second hologram.
The second photodetector may be divided into a plurality of portions substantially along a radius direction of the optical disk.
According to another aspect of the present invention, an optical pickup device is provided for performing recording and reproducing operations of information for both a first optical disk having a substrate of a first thickness and a second optical disk having a substrate of a second thickness which is thinner than the first thickness. The optical pickup device includes an objective lens and two hologram laser units each including a light source having a different wavelength. The objective lens is provided with an aperture which is formed based on a numerical aperture suitable for the second optical disk. A first hologram laser unit among the two hologram laser units to be used for performing recording and reproducing operations of information for the first optical disk includes a light source, a photodetector, and a hologram element which are formed integrally with each other. The hologram element includes: a first hologram for detecting information signals from the optical disk, the first hologram being provided in an area having an effective diameter corresponding to a numerical aperture suitable for the optical disk; and a second hologram for compensating for only a transmitted light amount of light traveling toward the optical disk from the light source, the second hologram being provided contiguously outside the first hologram.
The optical pickup device may further include a collimator lens, wherein the first hologram laser unit is disposed between the collimator lens and a focal point of the collimator lens.
According to the present invention, two types of holograms (i.e., a first hologram for detecting signals and a second hologram for compensating for the amount of transmitted light) are formed in a hologram element in a hologram laser unit. As a result, any redundant information other than that provided from an optical disk via a required numerical aperture is cut off by the second hologram (i.e., the compensation hologram) when the reflected light from the optical disk returning toward the hologram laser unit is diffracted by the hologram element. Thus, effects which would normally be obtained by providing an aperture can be effectively obtained without incorporating additional aperture.
By providing the second hologram outside the first hologram in a positionally contiguous manner, the variation in the light amount of a converged light spot on the optical disk can be minimized despite possible shifts in the position of the objective lens during tracking.
By providing the two types of holograms so that the respective diffraction directions thereof are substantially orthogonal with each other, the returning light diffracted by the second hologram (for compensating for the amount of transmitted light) can be prevented from becoming stray light that enters the photodetector of the hologram laser unit.
By providing the two types of holograms so that their respective pitches do not have substantial difference therebetween, there occurs no large difference between the amounts of transmitted light from the hologram laser unit into the optical disk, even when the two types of holograms are simultaneously processed. As a result, deterioration of the convergence characteristics on the optical disk can be prevented.
By providing an additional photodetector corresponding to the second hologram (for compensating for the amount of transmitted light) in the hologram laser unit, sufficient information signals can be secured even when reproducing an optical disk which calls for a larger numerical aperture than that is prescribed for the hologram laser unit.
By providing a photodetector corresponding to the second hologram so as to be divided into a plurality of portions substantially along the direction of a radius of an optical disk, it becomes possible to detect the shift amount of the objective lens.
Assume that an optical pickup device includes an object lens and two hologram units each having light sources of different wavelengths, and performs recording and reproducing operations of information for both a first optical disk having a substrate of a first thickness and a second optical disk having a substrate of a second thickness which is smaller than the first thickness. In such a case, by employing a hologram laser unit having the above-described features of the present invention as the first hologram laser unit to be used for the information recording/reproducing operations for the first optical disk, it becomes unnecessary to adjust the numerical aperture of the objective lens in accordance with the substrate thicknesses of optical disks during the recording and reproduction thereof.
By disposing one of the above-mentioned two hologram laser units to be used for optical disks with thicker substrates in a position between the collimator lens and the focal point thereof, the spherical aberration depending on the difference between the substrate thicknesses can be compensated for.
Thus, the invention described herein makes possible the advantages of (1) providing a hologram laser unit which can perform recording and reproducing operations of information for multiple types of optical disks having different substrate thicknesses with the use of the same objective lens by providing a variable numerical aperture, without degrading the optical performance or requiring a complicated mechanism or resulting in an increased cost, and which provides excellent mass-productivity and reliability; and (2) providing an optical pickup device incorporating such a hologram laser unit.
These and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures.